Seems to me that Dennis & Pensioner have it right - the critical
measurement is the outer circumference of a belted tire, not the
radius.  But ...

That's IFF (If and Only If) we can accurately compare the belted
radial tire to a tracked vehicle.  I wonder how valid that assumption
is; ie, can the effective belt length change significantly with
speed, load, pressure, etc.?  I'm guessing not but could be wrong.

//Jack